Free radical polymerization reactions typically have an initiator system. The initiator systems can be based on various chemical approaches. For example, free radical polymerization reactions can be initiated using a three-component photoinitiator system that includes an electron acceptor, an electron donor, and a sensitizing compound. Alternatively, an electron donor in combination with a sensitizing compound can be used as a photoinitiator system. Free radical polymerization reactions also can be initiated using a two-component thermal initiator system that includes an electron acceptor and an electron donor.
In thermally initiated free radical polymerization reactions, an electron donor usually reacts directly with an electron acceptor to reduce the electron acceptor. The reduced electron acceptor can be a radical that can function as an initiating free radical for the polymerization reaction. The reaction between the electron donor and acceptor can occur at room temperature or at elevated temperatures. The electron acceptor and the electron donor are often kept in separate containers (i.e., not mixed together) until polymerization is desired.
In contrast to thermally initiated systems, the components of a photoinitiator system usually can be mixed together prior to use. In a three-component photoinitiator system that includes an electron donor, electron acceptor and a sensitizing compound, there is typically no direct reaction between the electron donor and the electron acceptor. Rather, the sensitizing compound usually absorbs actinic radiation resulting in the formation of an excited sensitizing compound. The electron donor can donate an electron to the excited sensitizing compound. That is, the sensitizing compound is reduced and the electron donor is oxidized. The reduced sensitizing compound is a radical anion that can donate an electron to an electron acceptor to yield an initiating free radical for the polymerization reaction. The initiating free radical is the reduced electron acceptor. In some instances of a three-component photoinitiator system, the oxidized electron donor is a radical species that also can function as an initiating free radical.
Other photoinitiator systems include a sensitizing compound and an electron donor but no electron acceptor. The sensitizing compound can absorb actinic radiation to form an exited sensitizing compound. The electron donor can donate an electron to the excited sensitizing compound resulting in the oxidation of the electron donor. The oxidized electron donor is a radical species that functions as an initiating free radical for polymerization reactions.